A dopaminergic cell group is a crucial organization which is involved in movement control, hormone secretion control, and emotional control that are important in mammalian brains. Therefore, abnormalities in dopamine operability-mediated neuronal signaling cause various defects in nervous systems. For example, PD is a neurodegenerative disease of the extrapyramidal system, which is caused by specific degeneration of dopamine-producing neurons in substantia nigra. In terms of methods for treating PD, a method for orally administering 3,4-dihydroxyphenylalanine (i.e. levodopa) is generally carried out in order to supply reduced amount of produced dopamine, but sustainability of effects is known to be poor.
Recent methods for treating PD include an attempt to transplant midbrain ventral region of a 6 to 9 week-old aborted fetus including dopamine-producing neuronal progenitors in order to supply lost dopamine-producing neurons. However, at the current stage, this method has ethical and technical problems, such as requiring at least 5 to 10 fetal brain tissues for treating one patient, lacking effects on treatment in some cases, or causing side effects by overproduction of dopaminergic neural cells. Specifically, issues have been raised in various aspects, such as cell supply, ethical problems, danger of infection, contamination, immune rejection response to transplanted tissues, and low survival rate because of the fact that fetal tissues rely more on lipid metabolism than glycolysis.
In an attempt to solve problems regarding ethical aspects or supply shortage, for example, a method for using cerebral cortex, stria, and midbrain cells of pigs is suggested. The method requires complicated manipulations which change antigens on the cell surface (MHC class I antigen) to inhibit rejection response. For example, as far as methods for solving transplant rejection response are concerned, a method for locally inhibiting an immune response by transplanting Sertoli's cells at the same time is also suggested. It is possible to obtain cells for transplantation from relatives whose MHC is a match, bone marrow of others, a bone marrow bank, and a cord blood bank, but if patients' own cells can be used, the problem of rejection response can be solved without unnecessary manipulations. As such, using the dopaminergic neurons prepared in vitro from patients' own cells is considered to be promising.
Meanwhile, completely differentiated dopaminergic neurons have low settling rate in patients, and sustainable therapeutic effects are difficult to expect from the neurons. That is, reconstruction of brain function is required to treat injured neural tissues, and the cells, which have not been completely differentiated, and which may be differentiated specifically into the dopaminergic neurons in vivo, should be transplanted to form an appropriate link with surrounding cells (network formation). In the case where general neuronal stem cells are transplanted, there are not only ethical problems described above, but there are also problems involving possibility of differentiating transplanted stem cells into heterogeneous cell populations. Specifically, with respect to treatment for PD, it is required to selectively transplant the dopamine-producing neurons among the neurons containing catecholamine Up until now, cells for transplantation suggested to be used for treating PD are striatum, human fetal neuron-derived immortalized cell lines, human neurons produced after mitosis of NT2Z cells, primordial neural cells, the cells that are transfected by foreign genes to produce catecholamine such as dopamine, bone marrow stromal cells, ES cells in which the genes are manipulated, etc. In addition to this, it is also suggested to use the cells that express tyrosine hydroxylase by treating dopamine-producing neurons and NT2 neural cells with retinoic acid, wherein the dopamine-producing neurons and NT2 neural cells are formed by contacting fetal midbrain tissue-derived neural progenitors with FGF-8 and SHH. However, existing cells for transplantation have problems of heterogeneity because the ratio of dopamine-producing neurons or the ratio of the cells that are capable of being differentiated into the dopamine-producing neurons is not high within the existing cells for transplantation.
Specifically, among the methods above, one of the methods has been recently raised to solve immune rejection response by preparing patient-specific cells and to expect therapeutic effects via cell therapy by preparing necessary cells using pluripotent stem cells (embryonic stem cells or induced pluripotent stem cells). However, since the pluripotent stem cells can generally form teratoma, there is a danger of causing cancer when undifferentiated pluripotent stem cells are transplanted. Consequently, a reprogramming method called direct conversion (direct reprogramming or transdifferentiation) has been newly developed. Possibility of tumorigenesis in the induced pluripotent stem cells can be avoided because direct conversion to desired cells is possible without going through the induced pluripotent stem cell stage via direct conversion.
It has been reported that neural stem cells can be prepared by the direct conversion method at the current stage (refer to Kim, J. et al. Direct reprogramming of mouse fibroblasts to neural progenitors. Proc Natl Acad Sci USA, 2011) and the neural stem cells that are prepared in this manner are called the induced neural stem cells. The induced neural stem cells are predicted to be used for treatment and research on various neurological disorders.
At the current stage, one of the biggest problems of transplantation treatment for PD is that differentiation-inducing neural stem cells or neuronal progenitors are differentiated into numerous cell types because of low efficiency in differentiating into dopaminergic neurons. Considering safety of neural circuit formation, it is preferable to use the cells that can be differentiated into desired cell type at high rate. Therefore, it is preferable to transplant terminally differentiated dopaminergic neurons in order to prevent heterogeneity, but when considering cell survival and an ability to form an appropriate network in the transplanted position within the brain, it is preferable to use the neural progenitors having self-renewal ability. Accordingly, an increased therapeutic effect for treatment can be expected using the dopaminergic neural progenitors which have high efficiency in differentiating into the dopaminergic neurons.